The formation of a ka-tet is, of course, the completion of Roland's arc over the course of the film, and an important step in his maturation toward being the kind of man who can indeed save the Tower one day. In both the film and in the books, Roland is a loner when we meet him. He is committed to walking the world alone, and to finding Walter (though his motives vary slightly), and he has no real need for a companion other than for some temporary aid, like food or help in a fight or even, in the novels, a little sexual comfort. As time goes on, Roland becomes irrevocably tied to people, whether he likes it or not, and through learning to accept these connections he becomes a leader, a teacher, and a friend once again. It's only through doing these things, through growing personally, that he can enrich his life enough to make the rest of his quest possible. The final moments of The Dark Tower reveal him beginning to become this man, and while he perhaps has a very long way to go, it's a beginning.